OpenZFS 9290 - device removal reduces redundancy of mirrors

[mirror_zfs.git] / module / zfs / vdev_removal.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/spa_impl.h>
#include <sys/dmu.h>
#include <sys/dmu_tx.h>
#include <sys/zap.h>
#include <sys/vdev_impl.h>
#include <sys/metaslab.h>
#include <sys/metaslab_impl.h>
#include <sys/uberblock_impl.h>
#include <sys/txg.h>
#include <sys/avl.h>
#include <sys/bpobj.h>
#include <sys/dsl_pool.h>
#include <sys/dsl_synctask.h>
#include <sys/dsl_dir.h>
#include <sys/arc.h>
#include <sys/zfeature.h>
#include <sys/vdev_indirect_births.h>
#include <sys/vdev_indirect_mapping.h>
#include <sys/abd.h>
#include <sys/trace_vdev.h>

/*
 * This file contains the necessary logic to remove vdevs from a
 * storage pool.  Currently, the only devices that can be removed
 * are log, cache, and spare devices; and top level vdevs from a pool
 * w/o raidz or mirrors.  (Note that members of a mirror can be removed
 * by the detach operation.)
 *
 * Log vdevs are removed by evacuating them and then turning the vdev
 * into a hole vdev while holding spa config locks.
 *
 * Top level vdevs are removed and converted into an indirect vdev via
 * a multi-step process:
 *
 *  - Disable allocations from this device (spa_vdev_remove_top).
 *
 *  - From a new thread (spa_vdev_remove_thread), copy data from
 *    the removing vdev to a different vdev.  The copy happens in open
 *    context (spa_vdev_copy_impl) and issues a sync task
 *    (vdev_mapping_sync) so the sync thread can update the partial
 *    indirect mappings in core and on disk.
 *
 *  - If a free happens during a removal, it is freed from the
 *    removing vdev, and if it has already been copied, from the new
 *    location as well (free_from_removing_vdev).
 *
 *  - After the removal is completed, the copy thread converts the vdev
 *    into an indirect vdev (vdev_remove_complete) before instructing
 *    the sync thread to destroy the space maps and finish the removal
 *    (spa_finish_removal).
 */

typedef struct vdev_copy_arg {
        metaslab_t      *vca_msp;
        uint64_t        vca_outstanding_bytes;
        kcondvar_t      vca_cv;
        kmutex_t        vca_lock;
} vdev_copy_arg_t;

/*
 * The maximum amount of memory we can use for outstanding i/o while
 * doing a device removal.  This determines how much i/o we can have
 * in flight concurrently.
 */
int zfs_remove_max_copy_bytes = 64 * 1024 * 1024;

/*
 * The largest contiguous segment that we will attempt to allocate when
 * removing a device.  This can be no larger than SPA_MAXBLOCKSIZE.  If
 * there is a performance problem with attempting to allocate large blocks,
 * consider decreasing this.
 */
int zfs_remove_max_segment = SPA_MAXBLOCKSIZE;

#define VDEV_REMOVAL_ZAP_OBJS   "lzap"

static void spa_vdev_remove_thread(void *arg);

static void
spa_sync_removing_state(spa_t *spa, dmu_tx_t *tx)
{
        VERIFY0(zap_update(spa->spa_dsl_pool->dp_meta_objset,
            DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_REMOVING, sizeof (uint64_t),
            sizeof (spa->spa_removing_phys) / sizeof (uint64_t),
            &spa->spa_removing_phys, tx));
}

static nvlist_t *
spa_nvlist_lookup_by_guid(nvlist_t **nvpp, int count, uint64_t target_guid)
{
        for (int i = 0; i < count; i++) {
                uint64_t guid =
                    fnvlist_lookup_uint64(nvpp[i], ZPOOL_CONFIG_GUID);

                if (guid == target_guid)
                        return (nvpp[i]);
        }

        return (NULL);
}

static void
spa_vdev_remove_aux(nvlist_t *config, char *name, nvlist_t **dev, int count,
    nvlist_t *dev_to_remove)
{
        nvlist_t **newdev = NULL;

        if (count > 1)
                newdev = kmem_alloc((count - 1) * sizeof (void *), KM_SLEEP);

        for (int i = 0, j = 0; i < count; i++) {
                if (dev[i] == dev_to_remove)
                        continue;
                VERIFY(nvlist_dup(dev[i], &newdev[j++], KM_SLEEP) == 0);
        }

        VERIFY(nvlist_remove(config, name, DATA_TYPE_NVLIST_ARRAY) == 0);
        VERIFY(nvlist_add_nvlist_array(config, name, newdev, count - 1) == 0);

        for (int i = 0; i < count - 1; i++)
                nvlist_free(newdev[i]);

        if (count > 1)
                kmem_free(newdev, (count - 1) * sizeof (void *));
}

static spa_vdev_removal_t *
spa_vdev_removal_create(vdev_t *vd)
{
        spa_vdev_removal_t *svr = kmem_zalloc(sizeof (*svr), KM_SLEEP);
        mutex_init(&svr->svr_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&svr->svr_cv, NULL, CV_DEFAULT, NULL);
        svr->svr_allocd_segs = range_tree_create(NULL, NULL);
        svr->svr_vdev_id = vd->vdev_id;

        for (int i = 0; i < TXG_SIZE; i++) {
                svr->svr_frees[i] = range_tree_create(NULL, NULL);
                list_create(&svr->svr_new_segments[i],
                    sizeof (vdev_indirect_mapping_entry_t),
                    offsetof(vdev_indirect_mapping_entry_t, vime_node));
        }

        return (svr);
}

void
spa_vdev_removal_destroy(spa_vdev_removal_t *svr)
{
        for (int i = 0; i < TXG_SIZE; i++) {
                ASSERT0(svr->svr_bytes_done[i]);
                ASSERT0(svr->svr_max_offset_to_sync[i]);
                range_tree_destroy(svr->svr_frees[i]);
                list_destroy(&svr->svr_new_segments[i]);
        }

        range_tree_destroy(svr->svr_allocd_segs);
        mutex_destroy(&svr->svr_lock);
        cv_destroy(&svr->svr_cv);
        kmem_free(svr, sizeof (*svr));
}

/*
 * This is called as a synctask in the txg in which we will mark this vdev
 * as removing (in the config stored in the MOS).
 *
 * It begins the evacuation of a toplevel vdev by:
 * - initializing the spa_removing_phys which tracks this removal
 * - computing the amount of space to remove for accounting purposes
 * - dirtying all dbufs in the spa_config_object
 * - creating the spa_vdev_removal
 * - starting the spa_vdev_remove_thread
 */
static void
vdev_remove_initiate_sync(void *arg, dmu_tx_t *tx)
{
        int vdev_id = (uintptr_t)arg;
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;
        vdev_t *vd = vdev_lookup_top(spa, vdev_id);
        vdev_indirect_config_t *vic = &vd->vdev_indirect_config;
        objset_t *mos = spa->spa_dsl_pool->dp_meta_objset;
        spa_vdev_removal_t *svr = NULL;
        ASSERTV(uint64_t txg = dmu_tx_get_txg(tx));

        ASSERT3P(vd->vdev_ops, !=, &vdev_raidz_ops);
        svr = spa_vdev_removal_create(vd);

        ASSERT(vd->vdev_removing);
        ASSERT3P(vd->vdev_indirect_mapping, ==, NULL);

        spa_feature_incr(spa, SPA_FEATURE_DEVICE_REMOVAL, tx);
        if (spa_feature_is_enabled(spa, SPA_FEATURE_OBSOLETE_COUNTS)) {
                /*
                 * By activating the OBSOLETE_COUNTS feature, we prevent
                 * the pool from being downgraded and ensure that the
                 * refcounts are precise.
                 */
                spa_feature_incr(spa, SPA_FEATURE_OBSOLETE_COUNTS, tx);
                uint64_t one = 1;
                VERIFY0(zap_add(spa->spa_meta_objset, vd->vdev_top_zap,
                    VDEV_TOP_ZAP_OBSOLETE_COUNTS_ARE_PRECISE, sizeof (one), 1,
                    &one, tx));
                ASSERT3U(vdev_obsolete_counts_are_precise(vd), !=, 0);
        }

        vic->vic_mapping_object = vdev_indirect_mapping_alloc(mos, tx);
        vd->vdev_indirect_mapping =
            vdev_indirect_mapping_open(mos, vic->vic_mapping_object);
        vic->vic_births_object = vdev_indirect_births_alloc(mos, tx);
        vd->vdev_indirect_births =
            vdev_indirect_births_open(mos, vic->vic_births_object);
        spa->spa_removing_phys.sr_removing_vdev = vd->vdev_id;
        spa->spa_removing_phys.sr_start_time = gethrestime_sec();
        spa->spa_removing_phys.sr_end_time = 0;
        spa->spa_removing_phys.sr_state = DSS_SCANNING;
        spa->spa_removing_phys.sr_to_copy = 0;
        spa->spa_removing_phys.sr_copied = 0;

        /*
         * Note: We can't use vdev_stat's vs_alloc for sr_to_copy, because
         * there may be space in the defer tree, which is free, but still
         * counted in vs_alloc.
         */
        for (uint64_t i = 0; i < vd->vdev_ms_count; i++) {
                metaslab_t *ms = vd->vdev_ms[i];
                if (ms->ms_sm == NULL)
                        continue;

                /*
                 * Sync tasks happen before metaslab_sync(), therefore
                 * smp_alloc and sm_alloc must be the same.
                 */
                ASSERT3U(space_map_allocated(ms->ms_sm), ==,
                    ms->ms_sm->sm_phys->smp_alloc);

                spa->spa_removing_phys.sr_to_copy +=
                    space_map_allocated(ms->ms_sm);

                /*
                 * Space which we are freeing this txg does not need to
                 * be copied.
                 */
                spa->spa_removing_phys.sr_to_copy -=
                    range_tree_space(ms->ms_freeingtree);

                ASSERT0(range_tree_space(ms->ms_freedtree));
                for (int t = 0; t < TXG_SIZE; t++)
                        ASSERT0(range_tree_space(ms->ms_alloctree[t]));
        }

        /*
         * Sync tasks are called before metaslab_sync(), so there should
         * be no already-synced metaslabs in the TXG_CLEAN list.
         */
        ASSERT3P(txg_list_head(&vd->vdev_ms_list, TXG_CLEAN(txg)), ==, NULL);

        spa_sync_removing_state(spa, tx);

        /*
         * All blocks that we need to read the most recent mapping must be
         * stored on concrete vdevs.  Therefore, we must dirty anything that
         * is read before spa_remove_init().  Specifically, the
         * spa_config_object.  (Note that although we already modified the
         * spa_config_object in spa_sync_removing_state, that may not have
         * modified all blocks of the object.)
         */
        dmu_object_info_t doi;
        VERIFY0(dmu_object_info(mos, DMU_POOL_DIRECTORY_OBJECT, &doi));
        for (uint64_t offset = 0; offset < doi.doi_max_offset; ) {
                dmu_buf_t *dbuf;
                VERIFY0(dmu_buf_hold(mos, DMU_POOL_DIRECTORY_OBJECT,
                    offset, FTAG, &dbuf, 0));
                dmu_buf_will_dirty(dbuf, tx);
                offset += dbuf->db_size;
                dmu_buf_rele(dbuf, FTAG);
        }

        /*
         * Now that we've allocated the im_object, dirty the vdev to ensure
         * that the object gets written to the config on disk.
         */
        vdev_config_dirty(vd);

        zfs_dbgmsg("starting removal thread for vdev %llu (%p) in txg %llu "
            "im_obj=%llu", vd->vdev_id, vd, dmu_tx_get_txg(tx),
            vic->vic_mapping_object);

        spa_history_log_internal(spa, "vdev remove started", tx,
            "%s vdev %llu %s", spa_name(spa), vd->vdev_id,
            (vd->vdev_path != NULL) ? vd->vdev_path : "-");
        /*
         * Setting spa_vdev_removal causes subsequent frees to call
         * free_from_removing_vdev().  Note that we don't need any locking
         * because we are the sync thread, and metaslab_free_impl() is only
         * called from syncing context (potentially from a zio taskq thread,
         * but in any case only when there are outstanding free i/os, which
         * there are not).
         */
        ASSERT3P(spa->spa_vdev_removal, ==, NULL);
        spa->spa_vdev_removal = svr;
        svr->svr_thread = thread_create(NULL, 0,
            spa_vdev_remove_thread, spa, 0, &p0, TS_RUN, minclsyspri);
}

/*
 * When we are opening a pool, we must read the mapping for each
 * indirect vdev in order from most recently removed to least
 * recently removed.  We do this because the blocks for the mapping
 * of older indirect vdevs may be stored on more recently removed vdevs.
 * In order to read each indirect mapping object, we must have
 * initialized all more recently removed vdevs.
 */
int
spa_remove_init(spa_t *spa)
{
        int error;

        error = zap_lookup(spa->spa_dsl_pool->dp_meta_objset,
            DMU_POOL_DIRECTORY_OBJECT,
            DMU_POOL_REMOVING, sizeof (uint64_t),
            sizeof (spa->spa_removing_phys) / sizeof (uint64_t),
            &spa->spa_removing_phys);

        if (error == ENOENT) {
                spa->spa_removing_phys.sr_state = DSS_NONE;
                spa->spa_removing_phys.sr_removing_vdev = -1;
                spa->spa_removing_phys.sr_prev_indirect_vdev = -1;
                return (0);
        } else if (error != 0) {
                return (error);
        }

        if (spa->spa_removing_phys.sr_state == DSS_SCANNING) {
                /*
                 * We are currently removing a vdev.  Create and
                 * initialize a spa_vdev_removal_t from the bonus
                 * buffer of the removing vdevs vdev_im_object, and
                 * initialize its partial mapping.
                 */
                spa_config_enter(spa, SCL_STATE, FTAG, RW_READER);
                vdev_t *vd = vdev_lookup_top(spa,
                    spa->spa_removing_phys.sr_removing_vdev);

                if (vd == NULL) {
                        spa_config_exit(spa, SCL_STATE, FTAG);
                        return (EINVAL);
                }

                vdev_indirect_config_t *vic = &vd->vdev_indirect_config;

                ASSERT(vdev_is_concrete(vd));
                spa_vdev_removal_t *svr = spa_vdev_removal_create(vd);
                ASSERT3U(svr->svr_vdev_id, ==, vd->vdev_id);
                ASSERT(vd->vdev_removing);

                vd->vdev_indirect_mapping = vdev_indirect_mapping_open(
                    spa->spa_meta_objset, vic->vic_mapping_object);
                vd->vdev_indirect_births = vdev_indirect_births_open(
                    spa->spa_meta_objset, vic->vic_births_object);
                spa_config_exit(spa, SCL_STATE, FTAG);

                spa->spa_vdev_removal = svr;
        }

        spa_config_enter(spa, SCL_STATE, FTAG, RW_READER);
        uint64_t indirect_vdev_id =
            spa->spa_removing_phys.sr_prev_indirect_vdev;
        while (indirect_vdev_id != UINT64_MAX) {
                vdev_t *vd = vdev_lookup_top(spa, indirect_vdev_id);
                vdev_indirect_config_t *vic = &vd->vdev_indirect_config;

                ASSERT3P(vd->vdev_ops, ==, &vdev_indirect_ops);
                vd->vdev_indirect_mapping = vdev_indirect_mapping_open(
                    spa->spa_meta_objset, vic->vic_mapping_object);
                vd->vdev_indirect_births = vdev_indirect_births_open(
                    spa->spa_meta_objset, vic->vic_births_object);

                indirect_vdev_id = vic->vic_prev_indirect_vdev;
        }
        spa_config_exit(spa, SCL_STATE, FTAG);

        /*
         * Now that we've loaded all the indirect mappings, we can allow
         * reads from other blocks (e.g. via predictive prefetch).
         */
        spa->spa_indirect_vdevs_loaded = B_TRUE;
        return (0);
}

void
spa_restart_removal(spa_t *spa)
{
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;

        if (svr == NULL)
                return;

        /*
         * In general when this function is called there is no
         * removal thread running. The only scenario where this
         * is not true is during spa_import() where this function
         * is called twice [once from spa_import_impl() and
         * spa_async_resume()]. Thus, in the scenario where we
         * import a pool that has an ongoing removal we don't
         * want to spawn a second thread.
         */
        if (svr->svr_thread != NULL)
                return;

        if (!spa_writeable(spa))
                return;

        zfs_dbgmsg("restarting removal of %llu", svr->svr_vdev_id);
        svr->svr_thread = thread_create(NULL, 0, spa_vdev_remove_thread, spa,
            0, &p0, TS_RUN, minclsyspri);
}

/*
 * Process freeing from a device which is in the middle of being removed.
 * We must handle this carefully so that we attempt to copy freed data,
 * and we correctly free already-copied data.
 */
void
free_from_removing_vdev(vdev_t *vd, uint64_t offset, uint64_t size,
    uint64_t txg)
{
        spa_t *spa = vd->vdev_spa;
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;
        uint64_t max_offset_yet = 0;

        ASSERT(vd->vdev_indirect_config.vic_mapping_object != 0);
        ASSERT3U(vd->vdev_indirect_config.vic_mapping_object, ==,
            vdev_indirect_mapping_object(vim));
        ASSERT3U(vd->vdev_id, ==, svr->svr_vdev_id);
        ASSERT3U(spa_syncing_txg(spa), ==, txg);

        mutex_enter(&svr->svr_lock);

        /*
         * Remove the segment from the removing vdev's spacemap.  This
         * ensures that we will not attempt to copy this space (if the
         * removal thread has not yet visited it), and also ensures
         * that we know what is actually allocated on the new vdevs
         * (needed if we cancel the removal).
         *
         * Note: we must do the metaslab_free_concrete() with the svr_lock
         * held, so that the remove_thread can not load this metaslab and then
         * visit this offset between the time that we metaslab_free_concrete()
         * and when we check to see if it has been visited.
         */
        metaslab_free_concrete(vd, offset, size, txg);

        uint64_t synced_size = 0;
        uint64_t synced_offset = 0;
        uint64_t max_offset_synced = vdev_indirect_mapping_max_offset(vim);
        if (offset < max_offset_synced) {
                /*
                 * The mapping for this offset is already on disk.
                 * Free from the new location.
                 *
                 * Note that we use svr_max_synced_offset because it is
                 * updated atomically with respect to the in-core mapping.
                 * By contrast, vim_max_offset is not.
                 *
                 * This block may be split between a synced entry and an
                 * in-flight or unvisited entry.  Only process the synced
                 * portion of it here.
                 */
                synced_size = MIN(size, max_offset_synced - offset);
                synced_offset = offset;

                ASSERT3U(max_offset_yet, <=, max_offset_synced);
                max_offset_yet = max_offset_synced;

                DTRACE_PROBE3(remove__free__synced,
                    spa_t *, spa,
                    uint64_t, offset,
                    uint64_t, synced_size);

                size -= synced_size;
                offset += synced_size;
        }

        /*
         * Look at all in-flight txgs starting from the currently syncing one
         * and see if a section of this free is being copied. By starting from
         * this txg and iterating forward, we might find that this region
         * was copied in two different txgs and handle it appropriately.
         */
        for (int i = 0; i < TXG_CONCURRENT_STATES; i++) {
                int txgoff = (txg + i) & TXG_MASK;
                if (size > 0 && offset < svr->svr_max_offset_to_sync[txgoff]) {
                        /*
                         * The mapping for this offset is in flight, and
                         * will be synced in txg+i.
                         */
                        uint64_t inflight_size = MIN(size,
                            svr->svr_max_offset_to_sync[txgoff] - offset);

                        DTRACE_PROBE4(remove__free__inflight,
                            spa_t *, spa,
                            uint64_t, offset,
                            uint64_t, inflight_size,
                            uint64_t, txg + i);

                        /*
                         * We copy data in order of increasing offset.
                         * Therefore the max_offset_to_sync[] must increase
                         * (or be zero, indicating that nothing is being
                         * copied in that txg).
                         */
                        if (svr->svr_max_offset_to_sync[txgoff] != 0) {
                                ASSERT3U(svr->svr_max_offset_to_sync[txgoff],
                                    >=, max_offset_yet);
                                max_offset_yet =
                                    svr->svr_max_offset_to_sync[txgoff];
                        }

                        /*
                         * We've already committed to copying this segment:
                         * we have allocated space elsewhere in the pool for
                         * it and have an IO outstanding to copy the data. We
                         * cannot free the space before the copy has
                         * completed, or else the copy IO might overwrite any
                         * new data. To free that space, we record the
                         * segment in the appropriate svr_frees tree and free
                         * the mapped space later, in the txg where we have
                         * completed the copy and synced the mapping (see
                         * vdev_mapping_sync).
                         */
                        range_tree_add(svr->svr_frees[txgoff],
                            offset, inflight_size);
                        size -= inflight_size;
                        offset += inflight_size;

                        /*
                         * This space is already accounted for as being
                         * done, because it is being copied in txg+i.
                         * However, if i!=0, then it is being copied in
                         * a future txg.  If we crash after this txg
                         * syncs but before txg+i syncs, then the space
                         * will be free.  Therefore we must account
                         * for the space being done in *this* txg
                         * (when it is freed) rather than the future txg
                         * (when it will be copied).
                         */
                        ASSERT3U(svr->svr_bytes_done[txgoff], >=,
                            inflight_size);
                        svr->svr_bytes_done[txgoff] -= inflight_size;
                        svr->svr_bytes_done[txg & TXG_MASK] += inflight_size;
                }
        }
        ASSERT0(svr->svr_max_offset_to_sync[TXG_CLEAN(txg) & TXG_MASK]);

        if (size > 0) {
                /*
                 * The copy thread has not yet visited this offset.  Ensure
                 * that it doesn't.
                 */

                DTRACE_PROBE3(remove__free__unvisited,
                    spa_t *, spa,
                    uint64_t, offset,
                    uint64_t, size);

                if (svr->svr_allocd_segs != NULL)
                        range_tree_clear(svr->svr_allocd_segs, offset, size);

                /*
                 * Since we now do not need to copy this data, for
                 * accounting purposes we have done our job and can count
                 * it as completed.
                 */
                svr->svr_bytes_done[txg & TXG_MASK] += size;
        }
        mutex_exit(&svr->svr_lock);

        /*
         * Now that we have dropped svr_lock, process the synced portion
         * of this free.
         */
        if (synced_size > 0) {
                vdev_indirect_mark_obsolete(vd, synced_offset, synced_size,
                    txg);
                /*
                 * Note: this can only be called from syncing context,
                 * and the vdev_indirect_mapping is only changed from the
                 * sync thread, so we don't need svr_lock while doing
                 * metaslab_free_impl_cb.
                 */
                vdev_indirect_ops.vdev_op_remap(vd, synced_offset, synced_size,
                    metaslab_free_impl_cb, &txg);
        }
}

/*
 * Stop an active removal and update the spa_removing phys.
 */
static void
spa_finish_removal(spa_t *spa, dsl_scan_state_t state, dmu_tx_t *tx)
{
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        ASSERT3U(dmu_tx_get_txg(tx), ==, spa_syncing_txg(spa));

        /* Ensure the removal thread has completed before we free the svr. */
        spa_vdev_remove_suspend(spa);

        ASSERT(state == DSS_FINISHED || state == DSS_CANCELED);

        if (state == DSS_FINISHED) {
                spa_removing_phys_t *srp = &spa->spa_removing_phys;
                vdev_t *vd = vdev_lookup_top(spa, svr->svr_vdev_id);
                vdev_indirect_config_t *vic = &vd->vdev_indirect_config;

                if (srp->sr_prev_indirect_vdev != UINT64_MAX) {
                        vdev_t *pvd;
                        pvd = vdev_lookup_top(spa,
                            srp->sr_prev_indirect_vdev);
                        ASSERT3P(pvd->vdev_ops, ==, &vdev_indirect_ops);
                }

                vic->vic_prev_indirect_vdev = srp->sr_prev_indirect_vdev;
                srp->sr_prev_indirect_vdev = vd->vdev_id;
        }
        spa->spa_removing_phys.sr_state = state;
        spa->spa_removing_phys.sr_end_time = gethrestime_sec();

        spa->spa_vdev_removal = NULL;
        spa_vdev_removal_destroy(svr);

        spa_sync_removing_state(spa, tx);

        vdev_config_dirty(spa->spa_root_vdev);
}

static void
free_mapped_segment_cb(void *arg, uint64_t offset, uint64_t size)
{
        vdev_t *vd = arg;
        vdev_indirect_mark_obsolete(vd, offset, size,
            vd->vdev_spa->spa_syncing_txg);
        vdev_indirect_ops.vdev_op_remap(vd, offset, size,
            metaslab_free_impl_cb, &vd->vdev_spa->spa_syncing_txg);
}

/*
 * On behalf of the removal thread, syncs an incremental bit more of
 * the indirect mapping to disk and updates the in-memory mapping.
 * Called as a sync task in every txg that the removal thread makes progress.
 */
static void
vdev_mapping_sync(void *arg, dmu_tx_t *tx)
{
        spa_vdev_removal_t *svr = arg;
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;
        vdev_t *vd = vdev_lookup_top(spa, svr->svr_vdev_id);
        ASSERTV(vdev_indirect_config_t *vic = &vd->vdev_indirect_config);
        uint64_t txg = dmu_tx_get_txg(tx);
        vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;

        ASSERT(vic->vic_mapping_object != 0);
        ASSERT3U(txg, ==, spa_syncing_txg(spa));

        vdev_indirect_mapping_add_entries(vim,
            &svr->svr_new_segments[txg & TXG_MASK], tx);
        vdev_indirect_births_add_entry(vd->vdev_indirect_births,
            vdev_indirect_mapping_max_offset(vim), dmu_tx_get_txg(tx), tx);

        /*
         * Free the copied data for anything that was freed while the
         * mapping entries were in flight.
         */
        mutex_enter(&svr->svr_lock);
        range_tree_vacate(svr->svr_frees[txg & TXG_MASK],
            free_mapped_segment_cb, vd);
        ASSERT3U(svr->svr_max_offset_to_sync[txg & TXG_MASK], >=,
            vdev_indirect_mapping_max_offset(vim));
        svr->svr_max_offset_to_sync[txg & TXG_MASK] = 0;
        mutex_exit(&svr->svr_lock);

        spa_sync_removing_state(spa, tx);
}

/*
 * All reads and writes associated with a call to spa_vdev_copy_segment()
 * are done.
 */
static void
spa_vdev_copy_nullzio_done(zio_t *zio)
{
        spa_config_exit(zio->io_spa, SCL_STATE, zio->io_spa);
}

/*
 * The write of the new location is done.
 */
static void
spa_vdev_copy_segment_write_done(zio_t *zio)
{
        vdev_copy_arg_t *vca = zio->io_private;

        abd_free(zio->io_abd);

        mutex_enter(&vca->vca_lock);
        vca->vca_outstanding_bytes -= zio->io_size;
        cv_signal(&vca->vca_cv);
        mutex_exit(&vca->vca_lock);
}

/*
 * The read of the old location is done.  The parent zio is the write to
 * the new location.  Allow it to start.
 */
static void
spa_vdev_copy_segment_read_done(zio_t *zio)
{
        zio_nowait(zio_unique_parent(zio));
}

/*
 * If the old and new vdevs are mirrors, we will read both sides of the old
 * mirror, and write each copy to the corresponding side of the new mirror.
 * If the old and new vdevs have a different number of children, we will do
 * this as best as possible.  Since we aren't verifying checksums, this
 * ensures that as long as there's a good copy of the data, we'll have a
 * good copy after the removal, even if there's silent damage to one side
 * of the mirror. If we're removing a mirror that has some silent damage,
 * we'll have exactly the same damage in the new location (assuming that
 * the new location is also a mirror).
 *
 * We accomplish this by creating a tree of zio_t's, with as many writes as
 * there are "children" of the new vdev (a non-redundant vdev counts as one
 * child, a 2-way mirror has 2 children, etc). Each write has an associated
 * read from a child of the old vdev. Typically there will be the same
 * number of children of the old and new vdevs.  However, if there are more
 * children of the new vdev, some child(ren) of the old vdev will be issued
 * multiple reads.  If there are more children of the old vdev, some copies
 * will be dropped.
 *
 * For example, the tree of zio_t's for a 2-way mirror is:
 *
 *                            null
 *                           /    \
 *    write(new vdev, child 0)      write(new vdev, child 1)
 *      |                             |
 *    read(old vdev, child 0)       read(old vdev, child 1)
 *
 * Child zio's complete before their parents complete.  However, zio's
 * created with zio_vdev_child_io() may be issued before their children
 * complete.  In this case we need to make sure that the children (reads)
 * complete before the parents (writes) are *issued*.  We do this by not
 * calling zio_nowait() on each write until its corresponding read has
 * completed.
 *
 * The spa_config_lock must be held while zio's created by
 * zio_vdev_child_io() are in progress, to ensure that the vdev tree does
 * not change (e.g. due to a concurrent "zpool attach/detach"). The "null"
 * zio is needed to release the spa_config_lock after all the reads and
 * writes complete. (Note that we can't grab the config lock for each read,
 * because it is not reentrant - we could deadlock with a thread waiting
 * for a write lock.)
 */
static void
spa_vdev_copy_one_child(vdev_copy_arg_t *vca, zio_t *nzio,
    vdev_t *source_vd, uint64_t source_offset,
    vdev_t *dest_child_vd, uint64_t dest_offset, int dest_id, uint64_t size)
{
        ASSERT3U(spa_config_held(nzio->io_spa, SCL_ALL, RW_READER), !=, 0);

        mutex_enter(&vca->vca_lock);
        vca->vca_outstanding_bytes += size;
        mutex_exit(&vca->vca_lock);

        abd_t *abd = abd_alloc_for_io(size, B_FALSE);

        vdev_t *source_child_vd;
        if (source_vd->vdev_ops == &vdev_mirror_ops && dest_id != -1) {
                /*
                 * Source and dest are both mirrors.  Copy from the same
                 * child id as we are copying to (wrapping around if there
                 * are more dest children than source children).
                 */
                source_child_vd =
                    source_vd->vdev_child[dest_id % source_vd->vdev_children];
        } else {
                source_child_vd = source_vd;
        }

        zio_t *write_zio = zio_vdev_child_io(nzio, NULL,
            dest_child_vd, dest_offset, abd, size,
            ZIO_TYPE_WRITE, ZIO_PRIORITY_REMOVAL,
            ZIO_FLAG_CANFAIL,
            spa_vdev_copy_segment_write_done, vca);

        zio_nowait(zio_vdev_child_io(write_zio, NULL,
            source_child_vd, source_offset, abd, size,
            ZIO_TYPE_READ, ZIO_PRIORITY_REMOVAL,
            ZIO_FLAG_CANFAIL,
            spa_vdev_copy_segment_read_done, vca));
}

/*
 * Allocate a new location for this segment, and create the zio_t's to
 * read from the old location and write to the new location.
 */
static int
spa_vdev_copy_segment(vdev_t *vd, uint64_t start, uint64_t size, uint64_t txg,
    vdev_copy_arg_t *vca, zio_alloc_list_t *zal)
{
        metaslab_group_t *mg = vd->vdev_mg;
        spa_t *spa = vd->vdev_spa;
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        vdev_indirect_mapping_entry_t *entry;
        dva_t dst = {{ 0 }};

        ASSERT3U(size, <=, SPA_MAXBLOCKSIZE);

        int error = metaslab_alloc_dva(spa, mg->mg_class, size,
            &dst, 0, NULL, txg, 0, zal);
        if (error != 0)
                return (error);

        /*
         * We can't have any padding of the allocated size, otherwise we will
         * misunderstand what's allocated, and the size of the mapping.
         * The caller ensures this will be true by passing in a size that is
         * aligned to the worst (highest) ashift in the pool.
         */
        ASSERT3U(DVA_GET_ASIZE(&dst), ==, size);

        entry = kmem_zalloc(sizeof (vdev_indirect_mapping_entry_t), KM_SLEEP);
        DVA_MAPPING_SET_SRC_OFFSET(&entry->vime_mapping, start);
        entry->vime_mapping.vimep_dst = dst;

        /*
         * See comment before spa_vdev_copy_one_child().
         */
        spa_config_enter(spa, SCL_STATE, spa, RW_READER);
        zio_t *nzio = zio_null(spa->spa_txg_zio[txg & TXG_MASK], spa, NULL,
            spa_vdev_copy_nullzio_done, NULL, 0);
        vdev_t *dest_vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dst));
        if (dest_vd->vdev_ops == &vdev_mirror_ops) {
                for (int i = 0; i < dest_vd->vdev_children; i++) {
                        vdev_t *child = dest_vd->vdev_child[i];
                        spa_vdev_copy_one_child(vca, nzio, vd, start,
                            child, DVA_GET_OFFSET(&dst), i, size);
                }
        } else {
                spa_vdev_copy_one_child(vca, nzio, vd, start,
                    dest_vd, DVA_GET_OFFSET(&dst), -1, size);
        }
        zio_nowait(nzio);

        list_insert_tail(&svr->svr_new_segments[txg & TXG_MASK], entry);
        ASSERT3U(start + size, <=, vd->vdev_ms_count << vd->vdev_ms_shift);
        vdev_dirty(vd, 0, NULL, txg);

        return (0);
}

/*
 * Complete the removal of a toplevel vdev. This is called as a
 * synctask in the same txg that we will sync out the new config (to the
 * MOS object) which indicates that this vdev is indirect.
 */
static void
vdev_remove_complete_sync(void *arg, dmu_tx_t *tx)
{
        spa_vdev_removal_t *svr = arg;
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;
        vdev_t *vd = vdev_lookup_top(spa, svr->svr_vdev_id);

        ASSERT3P(vd->vdev_ops, ==, &vdev_indirect_ops);

        for (int i = 0; i < TXG_SIZE; i++) {
                ASSERT0(svr->svr_bytes_done[i]);
        }

        ASSERT3U(spa->spa_removing_phys.sr_copied, ==,
            spa->spa_removing_phys.sr_to_copy);

        vdev_destroy_spacemaps(vd, tx);

        /* destroy leaf zaps, if any */
        ASSERT3P(svr->svr_zaplist, !=, NULL);
        for (nvpair_t *pair = nvlist_next_nvpair(svr->svr_zaplist, NULL);
            pair != NULL;
            pair = nvlist_next_nvpair(svr->svr_zaplist, pair)) {
                vdev_destroy_unlink_zap(vd, fnvpair_value_uint64(pair), tx);
        }
        fnvlist_free(svr->svr_zaplist);

        spa_finish_removal(dmu_tx_pool(tx)->dp_spa, DSS_FINISHED, tx);
        /* vd->vdev_path is not available here */
        spa_history_log_internal(spa, "vdev remove completed",  tx,
            "%s vdev %llu", spa_name(spa), vd->vdev_id);
}

static void
vdev_remove_enlist_zaps(vdev_t *vd, nvlist_t *zlist)
{
        ASSERT3P(zlist, !=, NULL);
        ASSERT3P(vd->vdev_ops, !=, &vdev_raidz_ops);

        if (vd->vdev_leaf_zap != 0) {
                char zkey[32];
                (void) snprintf(zkey, sizeof (zkey), "%s-%llu",
                    VDEV_REMOVAL_ZAP_OBJS, (u_longlong_t)vd->vdev_leaf_zap);
                fnvlist_add_uint64(zlist, zkey, vd->vdev_leaf_zap);
        }

        for (uint64_t id = 0; id < vd->vdev_children; id++) {
                vdev_remove_enlist_zaps(vd->vdev_child[id], zlist);
        }
}

static void
vdev_remove_replace_with_indirect(vdev_t *vd, uint64_t txg)
{
        vdev_t *ivd;
        dmu_tx_t *tx;
        spa_t *spa = vd->vdev_spa;
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;

        /*
         * First, build a list of leaf zaps to be destroyed.
         * This is passed to the sync context thread,
         * which does the actual unlinking.
         */
        svr->svr_zaplist = fnvlist_alloc();
        vdev_remove_enlist_zaps(vd, svr->svr_zaplist);

        ivd = vdev_add_parent(vd, &vdev_indirect_ops);
        ivd->vdev_removing = 0;

        vd->vdev_leaf_zap = 0;

        vdev_remove_child(ivd, vd);
        vdev_compact_children(ivd);

        ASSERT(!list_link_active(&vd->vdev_state_dirty_node));

        tx = dmu_tx_create_assigned(spa->spa_dsl_pool, txg);
        dsl_sync_task_nowait(spa->spa_dsl_pool, vdev_remove_complete_sync, svr,
            0, ZFS_SPACE_CHECK_NONE, tx);
        dmu_tx_commit(tx);

        /*
         * Indicate that this thread has exited.
         * After this, we can not use svr.
         */
        mutex_enter(&svr->svr_lock);
        svr->svr_thread = NULL;
        cv_broadcast(&svr->svr_cv);
        mutex_exit(&svr->svr_lock);
}

/*
 * Complete the removal of a toplevel vdev. This is called in open
 * context by the removal thread after we have copied all vdev's data.
 */
static void
vdev_remove_complete(spa_t *spa)
{
        uint64_t txg;

        /*
         * Wait for any deferred frees to be synced before we call
         * vdev_metaslab_fini()
         */
        txg_wait_synced(spa->spa_dsl_pool, 0);
        txg = spa_vdev_enter(spa);
        vdev_t *vd = vdev_lookup_top(spa, spa->spa_vdev_removal->svr_vdev_id);

        sysevent_t *ev = spa_event_create(spa, vd, NULL,
            ESC_ZFS_VDEV_REMOVE_DEV);

        zfs_dbgmsg("finishing device removal for vdev %llu in txg %llu",
            vd->vdev_id, txg);

        /*
         * Discard allocation state.
         */
        if (vd->vdev_mg != NULL) {
                vdev_metaslab_fini(vd);
                metaslab_group_destroy(vd->vdev_mg);
                vd->vdev_mg = NULL;
        }
        ASSERT0(vd->vdev_stat.vs_space);
        ASSERT0(vd->vdev_stat.vs_dspace);

        vdev_remove_replace_with_indirect(vd, txg);

        /*
         * We now release the locks, allowing spa_sync to run and finish the
         * removal via vdev_remove_complete_sync in syncing context.
         *
         * Note that we hold on to the vdev_t that has been replaced.  Since
         * it isn't part of the vdev tree any longer, it can't be concurrently
         * manipulated, even while we don't have the config lock.
         */
        (void) spa_vdev_exit(spa, NULL, txg, 0);

        /*
         * Top ZAP should have been transferred to the indirect vdev in
         * vdev_remove_replace_with_indirect.
         */
        ASSERT0(vd->vdev_top_zap);

        /*
         * Leaf ZAP should have been moved in vdev_remove_replace_with_indirect.
         */
        ASSERT0(vd->vdev_leaf_zap);

        txg = spa_vdev_enter(spa);
        (void) vdev_label_init(vd, 0, VDEV_LABEL_REMOVE);
        /*
         * Request to update the config and the config cachefile.
         */
        vdev_config_dirty(spa->spa_root_vdev);
        (void) spa_vdev_exit(spa, vd, txg, 0);

        if (ev != NULL)
                spa_event_post(ev);
}

/*
 * Evacuates a segment of size at most max_alloc from the vdev
 * via repeated calls to spa_vdev_copy_segment. If an allocation
 * fails, the pool is probably too fragmented to handle such a
 * large size, so decrease max_alloc so that the caller will not try
 * this size again this txg.
 */
static void
spa_vdev_copy_impl(vdev_t *vd, spa_vdev_removal_t *svr, vdev_copy_arg_t *vca,
    uint64_t *max_alloc, dmu_tx_t *tx)
{
        uint64_t txg = dmu_tx_get_txg(tx);
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;

        mutex_enter(&svr->svr_lock);

        range_seg_t *rs = avl_first(&svr->svr_allocd_segs->rt_root);
        if (rs == NULL) {
                mutex_exit(&svr->svr_lock);
                return;
        }
        uint64_t offset = rs->rs_start;
        uint64_t length = MIN(rs->rs_end - rs->rs_start, *max_alloc);

        range_tree_remove(svr->svr_allocd_segs, offset, length);

        if (svr->svr_max_offset_to_sync[txg & TXG_MASK] == 0) {
                dsl_sync_task_nowait(dmu_tx_pool(tx), vdev_mapping_sync,
                    svr, 0, ZFS_SPACE_CHECK_NONE, tx);
        }

        svr->svr_max_offset_to_sync[txg & TXG_MASK] = offset + length;

        /*
         * Note: this is the amount of *allocated* space
         * that we are taking care of each txg.
         */
        svr->svr_bytes_done[txg & TXG_MASK] += length;

        mutex_exit(&svr->svr_lock);

        zio_alloc_list_t zal;
        metaslab_trace_init(&zal);
        uint64_t thismax = *max_alloc;
        while (length > 0) {
                uint64_t mylen = MIN(length, thismax);

                int error = spa_vdev_copy_segment(vd,
                    offset, mylen, txg, vca, &zal);

                if (error == ENOSPC) {
                        /*
                         * Cut our segment in half, and don't try this
                         * segment size again this txg.  Note that the
                         * allocation size must be aligned to the highest
                         * ashift in the pool, so that the allocation will
                         * not be padded out to a multiple of the ashift,
                         * which could cause us to think that this mapping
                         * is larger than we intended.
                         */
                        ASSERT3U(spa->spa_max_ashift, >=, SPA_MINBLOCKSHIFT);
                        ASSERT3U(spa->spa_max_ashift, ==, spa->spa_min_ashift);
                        thismax = P2ROUNDUP(mylen / 2,
                            1 << spa->spa_max_ashift);
                        ASSERT3U(thismax, <, mylen);
                        /*
                         * The minimum-size allocation can not fail.
                         */
                        ASSERT3U(mylen, >, 1 << spa->spa_max_ashift);
                        *max_alloc = mylen - (1 << spa->spa_max_ashift);
                } else {
                        ASSERT0(error);
                        length -= mylen;
                        offset += mylen;

                        /*
                         * We've performed an allocation, so reset the
                         * alloc trace list.
                         */
                        metaslab_trace_fini(&zal);
                        metaslab_trace_init(&zal);
                }
        }
        metaslab_trace_fini(&zal);
}

/*
 * The removal thread operates in open context.  It iterates over all
 * allocated space in the vdev, by loading each metaslab's spacemap.
 * For each contiguous segment of allocated space (capping the segment
 * size at SPA_MAXBLOCKSIZE), we:
 *    - Allocate space for it on another vdev.
 *    - Create a new mapping from the old location to the new location
 *      (as a record in svr_new_segments).
 *    - Initiate a physical read zio to get the data off the removing disk.
 *    - In the read zio's done callback, initiate a physical write zio to
 *      write it to the new vdev.
 * Note that all of this will take effect when a particular TXG syncs.
 * The sync thread ensures that all the phys reads and writes for the syncing
 * TXG have completed (see spa_txg_zio) and writes the new mappings to disk
 * (see vdev_mapping_sync()).
 */
static void
spa_vdev_remove_thread(void *arg)
{
        spa_t *spa = arg;
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        vdev_copy_arg_t vca;
        uint64_t max_alloc = zfs_remove_max_segment;
        uint64_t last_txg = 0;

        spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
        vdev_t *vd = vdev_lookup_top(spa, svr->svr_vdev_id);
        vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;
        uint64_t start_offset = vdev_indirect_mapping_max_offset(vim);

        ASSERT3P(vd->vdev_ops, !=, &vdev_indirect_ops);
        ASSERT(vdev_is_concrete(vd));
        ASSERT(vd->vdev_removing);
        ASSERT(vd->vdev_indirect_config.vic_mapping_object != 0);
        ASSERT(vim != NULL);

        mutex_init(&vca.vca_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&vca.vca_cv, NULL, CV_DEFAULT, NULL);
        vca.vca_outstanding_bytes = 0;

        mutex_enter(&svr->svr_lock);

        /*
         * Start from vim_max_offset so we pick up where we left off
         * if we are restarting the removal after opening the pool.
         */
        uint64_t msi;
        for (msi = start_offset >> vd->vdev_ms_shift;
            msi < vd->vdev_ms_count && !svr->svr_thread_exit; msi++) {
                metaslab_t *msp = vd->vdev_ms[msi];
                ASSERT3U(msi, <=, vd->vdev_ms_count);

                ASSERT0(range_tree_space(svr->svr_allocd_segs));

                mutex_enter(&msp->ms_sync_lock);
                mutex_enter(&msp->ms_lock);

                /*
                 * Assert nothing in flight -- ms_*tree is empty.
                 */
                for (int i = 0; i < TXG_SIZE; i++) {
                        ASSERT0(range_tree_space(msp->ms_alloctree[i]));
                }

                /*
                 * If the metaslab has ever been allocated from (ms_sm!=NULL),
                 * read the allocated segments from the space map object
                 * into svr_allocd_segs. Since we do this while holding
                 * svr_lock and ms_sync_lock, concurrent frees (which
                 * would have modified the space map) will wait for us
                 * to finish loading the spacemap, and then take the
                 * appropriate action (see free_from_removing_vdev()).
                 */
                if (msp->ms_sm != NULL) {
                        space_map_t *sm = NULL;

                        /*
                         * We have to open a new space map here, because
                         * ms_sm's sm_length and sm_alloc may not reflect
                         * what's in the object contents, if we are in between
                         * metaslab_sync() and metaslab_sync_done().
                         */
                        VERIFY0(space_map_open(&sm,
                            spa->spa_dsl_pool->dp_meta_objset,
                            msp->ms_sm->sm_object, msp->ms_sm->sm_start,
                            msp->ms_sm->sm_size, msp->ms_sm->sm_shift));
                        space_map_update(sm);
                        VERIFY0(space_map_load(sm, svr->svr_allocd_segs,
                            SM_ALLOC));
                        space_map_close(sm);

                        range_tree_walk(msp->ms_freeingtree,
                            range_tree_remove, svr->svr_allocd_segs);

                        /*
                         * When we are resuming from a paused removal (i.e.
                         * when importing a pool with a removal in progress),
                         * discard any state that we have already processed.
                         */
                        range_tree_clear(svr->svr_allocd_segs, 0, start_offset);
                }
                mutex_exit(&msp->ms_lock);
                mutex_exit(&msp->ms_sync_lock);

                vca.vca_msp = msp;
                zfs_dbgmsg("copying %llu segments for metaslab %llu",
                    avl_numnodes(&svr->svr_allocd_segs->rt_root),
                    msp->ms_id);

                while (!svr->svr_thread_exit &&
                    range_tree_space(svr->svr_allocd_segs) != 0) {

                        mutex_exit(&svr->svr_lock);

                        /*
                         * We need to periodically drop the config lock so that
                         * writers can get in.  Additionally, we can't wait
                         * for a txg to sync while holding a config lock
                         * (since a waiting writer could cause a 3-way deadlock
                         * with the sync thread, which also gets a config
                         * lock for reader).  So we can't hold the config lock
                         * while calling dmu_tx_assign().
                         */
                        spa_config_exit(spa, SCL_CONFIG, FTAG);

                        mutex_enter(&vca.vca_lock);
                        while (vca.vca_outstanding_bytes >
                            zfs_remove_max_copy_bytes) {
                                cv_wait(&vca.vca_cv, &vca.vca_lock);
                        }
                        mutex_exit(&vca.vca_lock);

                        dmu_tx_t *tx =
                            dmu_tx_create_dd(spa_get_dsl(spa)->dp_mos_dir);
                        dmu_tx_hold_space(tx, SPA_MAXBLOCKSIZE);
                        VERIFY0(dmu_tx_assign(tx, TXG_WAIT));
                        uint64_t txg = dmu_tx_get_txg(tx);

                        /*
                         * Reacquire the vdev_config lock.  The vdev_t
                         * that we're removing may have changed, e.g. due
                         * to a vdev_attach or vdev_detach.
                         */
                        spa_config_enter(spa, SCL_CONFIG, FTAG, RW_READER);
                        vd = vdev_lookup_top(spa, svr->svr_vdev_id);

                        if (txg != last_txg)
                                max_alloc = zfs_remove_max_segment;
                        last_txg = txg;

                        spa_vdev_copy_impl(vd, svr, &vca, &max_alloc, tx);

                        dmu_tx_commit(tx);
                        mutex_enter(&svr->svr_lock);
                }
        }

        mutex_exit(&svr->svr_lock);

        spa_config_exit(spa, SCL_CONFIG, FTAG);

        /*
         * Wait for all copies to finish before cleaning up the vca.
         */
        txg_wait_synced(spa->spa_dsl_pool, 0);
        ASSERT0(vca.vca_outstanding_bytes);

        mutex_destroy(&vca.vca_lock);
        cv_destroy(&vca.vca_cv);

        if (svr->svr_thread_exit) {
                mutex_enter(&svr->svr_lock);
                range_tree_vacate(svr->svr_allocd_segs, NULL, NULL);
                svr->svr_thread = NULL;
                cv_broadcast(&svr->svr_cv);
                mutex_exit(&svr->svr_lock);
        } else {
                ASSERT0(range_tree_space(svr->svr_allocd_segs));
                vdev_remove_complete(spa);
        }
}

void
spa_vdev_remove_suspend(spa_t *spa)
{
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;

        if (svr == NULL)
                return;

        mutex_enter(&svr->svr_lock);
        svr->svr_thread_exit = B_TRUE;
        while (svr->svr_thread != NULL)
                cv_wait(&svr->svr_cv, &svr->svr_lock);
        svr->svr_thread_exit = B_FALSE;
        mutex_exit(&svr->svr_lock);
}

/* ARGSUSED */
static int
spa_vdev_remove_cancel_check(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;

        if (spa->spa_vdev_removal == NULL)
                return (ENOTACTIVE);
        return (0);
}

/*
 * Cancel a removal by freeing all entries from the partial mapping
 * and marking the vdev as no longer being removing.
 */
/* ARGSUSED */
static void
spa_vdev_remove_cancel_sync(void *arg, dmu_tx_t *tx)
{
        spa_t *spa = dmu_tx_pool(tx)->dp_spa;
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        vdev_t *vd = vdev_lookup_top(spa, svr->svr_vdev_id);
        vdev_indirect_config_t *vic = &vd->vdev_indirect_config;
        vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;
        objset_t *mos = spa->spa_meta_objset;

        ASSERT3P(svr->svr_thread, ==, NULL);

        spa_feature_decr(spa, SPA_FEATURE_DEVICE_REMOVAL, tx);
        if (vdev_obsolete_counts_are_precise(vd)) {
                spa_feature_decr(spa, SPA_FEATURE_OBSOLETE_COUNTS, tx);
                VERIFY0(zap_remove(spa->spa_meta_objset, vd->vdev_top_zap,
                    VDEV_TOP_ZAP_OBSOLETE_COUNTS_ARE_PRECISE, tx));
        }

        if (vdev_obsolete_sm_object(vd) != 0) {
                ASSERT(vd->vdev_obsolete_sm != NULL);
                ASSERT3U(vdev_obsolete_sm_object(vd), ==,
                    space_map_object(vd->vdev_obsolete_sm));

                space_map_free(vd->vdev_obsolete_sm, tx);
                VERIFY0(zap_remove(spa->spa_meta_objset, vd->vdev_top_zap,
                    VDEV_TOP_ZAP_INDIRECT_OBSOLETE_SM, tx));
                space_map_close(vd->vdev_obsolete_sm);
                vd->vdev_obsolete_sm = NULL;
                spa_feature_decr(spa, SPA_FEATURE_OBSOLETE_COUNTS, tx);
        }
        for (int i = 0; i < TXG_SIZE; i++) {
                ASSERT(list_is_empty(&svr->svr_new_segments[i]));
                ASSERT3U(svr->svr_max_offset_to_sync[i], <=,
                    vdev_indirect_mapping_max_offset(vim));
        }

        for (uint64_t msi = 0; msi < vd->vdev_ms_count; msi++) {
                metaslab_t *msp = vd->vdev_ms[msi];

                if (msp->ms_start >= vdev_indirect_mapping_max_offset(vim))
                        break;

                ASSERT0(range_tree_space(svr->svr_allocd_segs));

                mutex_enter(&msp->ms_lock);

                /*
                 * Assert nothing in flight -- ms_*tree is empty.
                 */
                for (int i = 0; i < TXG_SIZE; i++)
                        ASSERT0(range_tree_space(msp->ms_alloctree[i]));
                for (int i = 0; i < TXG_DEFER_SIZE; i++)
                        ASSERT0(range_tree_space(msp->ms_defertree[i]));
                ASSERT0(range_tree_space(msp->ms_freedtree));

                if (msp->ms_sm != NULL) {
                        /*
                         * Assert that the in-core spacemap has the same
                         * length as the on-disk one, so we can use the
                         * existing in-core spacemap to load it from disk.
                         */
                        ASSERT3U(msp->ms_sm->sm_alloc, ==,
                            msp->ms_sm->sm_phys->smp_alloc);
                        ASSERT3U(msp->ms_sm->sm_length, ==,
                            msp->ms_sm->sm_phys->smp_objsize);

                        mutex_enter(&svr->svr_lock);
                        VERIFY0(space_map_load(msp->ms_sm,
                            svr->svr_allocd_segs, SM_ALLOC));
                        range_tree_walk(msp->ms_freeingtree,
                            range_tree_remove, svr->svr_allocd_segs);

                        /*
                         * Clear everything past what has been synced,
                         * because we have not allocated mappings for it yet.
                         */
                        uint64_t syncd = vdev_indirect_mapping_max_offset(vim);
                        uint64_t sm_end = msp->ms_sm->sm_start +
                            msp->ms_sm->sm_size;
                        if (sm_end > syncd)
                                range_tree_clear(svr->svr_allocd_segs,
                                    syncd, sm_end - syncd);

                        mutex_exit(&svr->svr_lock);
                }
                mutex_exit(&msp->ms_lock);

                mutex_enter(&svr->svr_lock);
                range_tree_vacate(svr->svr_allocd_segs,
                    free_mapped_segment_cb, vd);
                mutex_exit(&svr->svr_lock);
        }

        /*
         * Note: this must happen after we invoke free_mapped_segment_cb,
         * because it adds to the obsolete_segments.
         */
        range_tree_vacate(vd->vdev_obsolete_segments, NULL, NULL);

        ASSERT3U(vic->vic_mapping_object, ==,
            vdev_indirect_mapping_object(vd->vdev_indirect_mapping));
        vdev_indirect_mapping_close(vd->vdev_indirect_mapping);
        vd->vdev_indirect_mapping = NULL;
        vdev_indirect_mapping_free(mos, vic->vic_mapping_object, tx);
        vic->vic_mapping_object = 0;

        ASSERT3U(vic->vic_births_object, ==,
            vdev_indirect_births_object(vd->vdev_indirect_births));
        vdev_indirect_births_close(vd->vdev_indirect_births);
        vd->vdev_indirect_births = NULL;
        vdev_indirect_births_free(mos, vic->vic_births_object, tx);
        vic->vic_births_object = 0;

        /*
         * We may have processed some frees from the removing vdev in this
         * txg, thus increasing svr_bytes_done; discard that here to
         * satisfy the assertions in spa_vdev_removal_destroy().
         * Note that future txg's can not have any bytes_done, because
         * future TXG's are only modified from open context, and we have
         * already shut down the copying thread.
         */
        svr->svr_bytes_done[dmu_tx_get_txg(tx) & TXG_MASK] = 0;
        spa_finish_removal(spa, DSS_CANCELED, tx);

        vd->vdev_removing = B_FALSE;
        vdev_config_dirty(vd);

        zfs_dbgmsg("canceled device removal for vdev %llu in %llu",
            vd->vdev_id, dmu_tx_get_txg(tx));
        spa_history_log_internal(spa, "vdev remove canceled", tx,
            "%s vdev %llu %s", spa_name(spa),
            vd->vdev_id, (vd->vdev_path != NULL) ? vd->vdev_path : "-");
}

int
spa_vdev_remove_cancel(spa_t *spa)
{
        spa_vdev_remove_suspend(spa);

        if (spa->spa_vdev_removal == NULL)
                return (ENOTACTIVE);

        uint64_t vdid = spa->spa_vdev_removal->svr_vdev_id;

        int error = dsl_sync_task(spa->spa_name, spa_vdev_remove_cancel_check,
            spa_vdev_remove_cancel_sync, NULL, 0, ZFS_SPACE_CHECK_NONE);

        if (error == 0) {
                spa_config_enter(spa, SCL_ALLOC | SCL_VDEV, FTAG, RW_WRITER);
                vdev_t *vd = vdev_lookup_top(spa, vdid);
                metaslab_group_activate(vd->vdev_mg);
                spa_config_exit(spa, SCL_ALLOC | SCL_VDEV, FTAG);
        }

        return (error);
}

/*
 * Called every sync pass of every txg if there's a svr.
 */
void
svr_sync(spa_t *spa, dmu_tx_t *tx)
{
        spa_vdev_removal_t *svr = spa->spa_vdev_removal;
        int txgoff = dmu_tx_get_txg(tx) & TXG_MASK;

        /*
         * This check is necessary so that we do not dirty the
         * DIRECTORY_OBJECT via spa_sync_removing_state() when there
         * is nothing to do.  Dirtying it every time would prevent us
         * from syncing-to-convergence.
         */
        if (svr->svr_bytes_done[txgoff] == 0)
                return;

        /*
         * Update progress accounting.
         */
        spa->spa_removing_phys.sr_copied += svr->svr_bytes_done[txgoff];
        svr->svr_bytes_done[txgoff] = 0;

        spa_sync_removing_state(spa, tx);
}

static void
vdev_remove_make_hole_and_free(vdev_t *vd)
{
        uint64_t id = vd->vdev_id;
        spa_t *spa = vd->vdev_spa;
        vdev_t *rvd = spa->spa_root_vdev;
        boolean_t last_vdev = (id == (rvd->vdev_children - 1));

        ASSERT(MUTEX_HELD(&spa_namespace_lock));
        ASSERT(spa_config_held(spa, SCL_ALL, RW_WRITER) == SCL_ALL);

        vdev_free(vd);

        if (last_vdev) {
                vdev_compact_children(rvd);
        } else {
                vd = vdev_alloc_common(spa, id, 0, &vdev_hole_ops);
                vdev_add_child(rvd, vd);
        }
        vdev_config_dirty(rvd);

        /*
         * Reassess the health of our root vdev.
         */
        vdev_reopen(rvd);
}

/*
 * Remove a log device.  The config lock is held for the specified TXG.
 */
static int
spa_vdev_remove_log(vdev_t *vd, uint64_t *txg)
{
        metaslab_group_t *mg = vd->vdev_mg;
        spa_t *spa = vd->vdev_spa;
        int error = 0;

        ASSERT(vd->vdev_islog);
        ASSERT(vd == vd->vdev_top);

        /*
         * Stop allocating from this vdev.
         */
        metaslab_group_passivate(mg);

        /*
         * Wait for the youngest allocations and frees to sync,
         * and then wait for the deferral of those frees to finish.
         */
        spa_vdev_config_exit(spa, NULL,
            *txg + TXG_CONCURRENT_STATES + TXG_DEFER_SIZE, 0, FTAG);

        /*
         * Evacuate the device.  We don't hold the config lock as writer
         * since we need to do I/O but we do keep the
         * spa_namespace_lock held.  Once this completes the device
         * should no longer have any blocks allocated on it.
         */
        if (vd->vdev_islog) {
                if (vd->vdev_stat.vs_alloc != 0)
                        error = spa_reset_logs(spa);
        }

        *txg = spa_vdev_config_enter(spa);

        if (error != 0) {
                metaslab_group_activate(mg);
                return (error);
        }
        ASSERT0(vd->vdev_stat.vs_alloc);

        /*
         * The evacuation succeeded.  Remove any remaining MOS metadata
         * associated with this vdev, and wait for these changes to sync.
         */
        vd->vdev_removing = B_TRUE;

        vdev_dirty_leaves(vd, VDD_DTL, *txg);
        vdev_config_dirty(vd);

        spa_history_log_internal(spa, "vdev remove", NULL,
            "%s vdev %llu (log) %s", spa_name(spa), vd->vdev_id,
            (vd->vdev_path != NULL) ? vd->vdev_path : "-");

        spa_vdev_config_exit(spa, NULL, *txg, 0, FTAG);

        *txg = spa_vdev_config_enter(spa);

        sysevent_t *ev = spa_event_create(spa, vd, NULL,
            ESC_ZFS_VDEV_REMOVE_DEV);
        ASSERT(MUTEX_HELD(&spa_namespace_lock));
        ASSERT(spa_config_held(spa, SCL_ALL, RW_WRITER) == SCL_ALL);

        /* The top ZAP should have been destroyed by vdev_remove_empty. */
        ASSERT0(vd->vdev_top_zap);
        /* The leaf ZAP should have been destroyed by vdev_dtl_sync. */
        ASSERT0(vd->vdev_leaf_zap);

        (void) vdev_label_init(vd, 0, VDEV_LABEL_REMOVE);

        if (list_link_active(&vd->vdev_state_dirty_node))
                vdev_state_clean(vd);
        if (list_link_active(&vd->vdev_config_dirty_node))
                vdev_config_clean(vd);

        /*
         * Clean up the vdev namespace.
         */
        vdev_remove_make_hole_and_free(vd);

        if (ev != NULL)
                spa_event_post(ev);

        return (0);
}

static int
spa_vdev_remove_top_check(vdev_t *vd)
{
        spa_t *spa = vd->vdev_spa;

        if (vd != vd->vdev_top)
                return (SET_ERROR(ENOTSUP));

        if (!spa_feature_is_enabled(spa, SPA_FEATURE_DEVICE_REMOVAL))
                return (SET_ERROR(ENOTSUP));

        /*
         * There has to be enough free space to remove the
         * device and leave double the "slop" space (i.e. we
         * must leave at least 3% of the pool free, in addition to
         * the normal slop space).
         */
        if (dsl_dir_space_available(spa->spa_dsl_pool->dp_root_dir,
            NULL, 0, B_TRUE) <
            vd->vdev_stat.vs_dspace + spa_get_slop_space(spa)) {
                return (SET_ERROR(ENOSPC));
        }

        /*
         * There can not be a removal in progress.
         */
        if (spa->spa_removing_phys.sr_state == DSS_SCANNING)
                return (SET_ERROR(EBUSY));

        /*
         * The device must have all its data.
         */
        if (!vdev_dtl_empty(vd, DTL_MISSING) ||
            !vdev_dtl_empty(vd, DTL_OUTAGE))
                return (SET_ERROR(EBUSY));

        /*
         * The device must be healthy.
         */
        if (!vdev_readable(vd))
                return (SET_ERROR(EIO));

        /*
         * All vdevs in normal class must have the same ashift.
         */
        if (spa->spa_max_ashift != spa->spa_min_ashift) {
                return (SET_ERROR(EINVAL));
        }

        /*
         * All vdevs in normal class must have the same ashift
         * and not be raidz.
         */
        vdev_t *rvd = spa->spa_root_vdev;
        int num_indirect = 0;
        for (uint64_t id = 0; id < rvd->vdev_children; id++) {
                vdev_t *cvd = rvd->vdev_child[id];
                if (cvd->vdev_ashift != 0 && !cvd->vdev_islog)
                        ASSERT3U(cvd->vdev_ashift, ==, spa->spa_max_ashift);
                if (cvd->vdev_ops == &vdev_indirect_ops)
                        num_indirect++;
                if (!vdev_is_concrete(cvd))
                        continue;
                if (cvd->vdev_ops == &vdev_raidz_ops)
                        return (SET_ERROR(EINVAL));
                /*
                 * Need the mirror to be mirror of leaf vdevs only
                 */
                if (cvd->vdev_ops == &vdev_mirror_ops) {
                        for (uint64_t cid = 0;
                            cid < cvd->vdev_children; cid++) {
                                if (!cvd->vdev_child[cid]->vdev_ops->
                                    vdev_op_leaf)
                                        return (SET_ERROR(EINVAL));
                        }
                }
        }

        return (0);
}

/*
 * Initiate removal of a top-level vdev, reducing the total space in the pool.
 * The config lock is held for the specified TXG.  Once initiated,
 * evacuation of all allocated space (copying it to other vdevs) happens
 * in the background (see spa_vdev_remove_thread()), and can be canceled
 * (see spa_vdev_remove_cancel()).  If successful, the vdev will
 * be transformed to an indirect vdev (see spa_vdev_remove_complete()).
 */
static int
spa_vdev_remove_top(vdev_t *vd, uint64_t *txg)
{
        spa_t *spa = vd->vdev_spa;
        int error;

        /*
         * Check for errors up-front, so that we don't waste time
         * passivating the metaslab group and clearing the ZIL if there
         * are errors.
         */
        error = spa_vdev_remove_top_check(vd);
        if (error != 0)
                return (error);

        /*
         * Stop allocating from this vdev.  Note that we must check
         * that this is not the only device in the pool before
         * passivating, otherwise we will not be able to make
         * progress because we can't allocate from any vdevs.
         * The above check for sufficient free space serves this
         * purpose.
         */
        metaslab_group_t *mg = vd->vdev_mg;
        metaslab_group_passivate(mg);

        /*
         * Wait for the youngest allocations and frees to sync,
         * and then wait for the deferral of those frees to finish.
         */
        spa_vdev_config_exit(spa, NULL,
            *txg + TXG_CONCURRENT_STATES + TXG_DEFER_SIZE, 0, FTAG);

        /*
         * We must ensure that no "stubby" log blocks are allocated
         * on the device to be removed.  These blocks could be
         * written at any time, including while we are in the middle
         * of copying them.
         */
        error = spa_reset_logs(spa);

        *txg = spa_vdev_config_enter(spa);

        /*
         * Things might have changed while the config lock was dropped
         * (e.g. space usage).  Check for errors again.
         */
        if (error == 0)
                error = spa_vdev_remove_top_check(vd);

        if (error != 0) {
                metaslab_group_activate(mg);
                return (error);
        }

        vd->vdev_removing = B_TRUE;

        vdev_dirty_leaves(vd, VDD_DTL, *txg);
        vdev_config_dirty(vd);
        dmu_tx_t *tx = dmu_tx_create_assigned(spa->spa_dsl_pool, *txg);
        dsl_sync_task_nowait(spa->spa_dsl_pool,
            vdev_remove_initiate_sync,
            (void *)(uintptr_t)vd->vdev_id, 0, ZFS_SPACE_CHECK_NONE, tx);
        dmu_tx_commit(tx);

        return (0);
}

/*
 * Remove a device from the pool.
 *
 * Removing a device from the vdev namespace requires several steps
 * and can take a significant amount of time.  As a result we use
 * the spa_vdev_config_[enter/exit] functions which allow us to
 * grab and release the spa_config_lock while still holding the namespace
 * lock.  During each step the configuration is synced out.
 */
int
spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
{
        vdev_t *vd;
        nvlist_t **spares, **l2cache, *nv;
        uint64_t txg = 0;
        uint_t nspares, nl2cache;
        int error = 0;
        boolean_t locked = MUTEX_HELD(&spa_namespace_lock);
        sysevent_t *ev = NULL;

        ASSERT(spa_writeable(spa));

        if (!locked)
                txg = spa_vdev_enter(spa);

        vd = spa_lookup_by_guid(spa, guid, B_FALSE);

        if (spa->spa_spares.sav_vdevs != NULL &&
            nvlist_lookup_nvlist_array(spa->spa_spares.sav_config,
            ZPOOL_CONFIG_SPARES, &spares, &nspares) == 0 &&
            (nv = spa_nvlist_lookup_by_guid(spares, nspares, guid)) != NULL) {
                /*
                 * Only remove the hot spare if it's not currently in use
                 * in this pool.
                 */
                if (vd == NULL || unspare) {
                        if (vd == NULL)
                                vd = spa_lookup_by_guid(spa, guid, B_TRUE);
                        ev = spa_event_create(spa, vd, NULL,
                            ESC_ZFS_VDEV_REMOVE_AUX);

                        char *nvstr = fnvlist_lookup_string(nv,
                            ZPOOL_CONFIG_PATH);
                        spa_history_log_internal(spa, "vdev remove", NULL,
                            "%s vdev (%s) %s", spa_name(spa),
                            VDEV_TYPE_SPARE, nvstr);
                        spa_vdev_remove_aux(spa->spa_spares.sav_config,
                            ZPOOL_CONFIG_SPARES, spares, nspares, nv);
                        spa_load_spares(spa);
                        spa->spa_spares.sav_sync = B_TRUE;
                } else {
                        error = SET_ERROR(EBUSY);
                }
        } else if (spa->spa_l2cache.sav_vdevs != NULL &&
            nvlist_lookup_nvlist_array(spa->spa_l2cache.sav_config,
            ZPOOL_CONFIG_L2CACHE, &l2cache, &nl2cache) == 0 &&
            (nv = spa_nvlist_lookup_by_guid(l2cache, nl2cache, guid)) != NULL) {
                char *nvstr = fnvlist_lookup_string(nv, ZPOOL_CONFIG_PATH);
                spa_history_log_internal(spa, "vdev remove", NULL,
                    "%s vdev (%s) %s", spa_name(spa), VDEV_TYPE_L2CACHE, nvstr);
                /*
                 * Cache devices can always be removed.
                 */
                vd = spa_lookup_by_guid(spa, guid, B_TRUE);
                ev = spa_event_create(spa, vd, NULL, ESC_ZFS_VDEV_REMOVE_AUX);
                spa_vdev_remove_aux(spa->spa_l2cache.sav_config,
                    ZPOOL_CONFIG_L2CACHE, l2cache, nl2cache, nv);
                spa_load_l2cache(spa);
                spa->spa_l2cache.sav_sync = B_TRUE;
        } else if (vd != NULL && vd->vdev_islog) {
                ASSERT(!locked);
                error = spa_vdev_remove_log(vd, &txg);
        } else if (vd != NULL) {
                ASSERT(!locked);
                error = spa_vdev_remove_top(vd, &txg);
        } else {
                /*
                 * There is no vdev of any kind with the specified guid.
                 */
                error = SET_ERROR(ENOENT);
        }

        if (!locked)
                error = spa_vdev_exit(spa, NULL, txg, error);

        if (ev != NULL)
                spa_event_post(ev);

        return (error);
}

int
spa_removal_get_stats(spa_t *spa, pool_removal_stat_t *prs)
{
        prs->prs_state = spa->spa_removing_phys.sr_state;

        if (prs->prs_state == DSS_NONE)
                return (SET_ERROR(ENOENT));

        prs->prs_removing_vdev = spa->spa_removing_phys.sr_removing_vdev;
        prs->prs_start_time = spa->spa_removing_phys.sr_start_time;
        prs->prs_end_time = spa->spa_removing_phys.sr_end_time;
        prs->prs_to_copy = spa->spa_removing_phys.sr_to_copy;
        prs->prs_copied = spa->spa_removing_phys.sr_copied;

        if (spa->spa_vdev_removal != NULL) {
                for (int i = 0; i < TXG_SIZE; i++) {
                        prs->prs_copied +=
                            spa->spa_vdev_removal->svr_bytes_done[i];
                }
        }

        prs->prs_mapping_memory = 0;
        uint64_t indirect_vdev_id =
            spa->spa_removing_phys.sr_prev_indirect_vdev;
        while (indirect_vdev_id != -1) {
                vdev_t *vd = spa->spa_root_vdev->vdev_child[indirect_vdev_id];
                vdev_indirect_config_t *vic = &vd->vdev_indirect_config;
                vdev_indirect_mapping_t *vim = vd->vdev_indirect_mapping;

                ASSERT3P(vd->vdev_ops, ==, &vdev_indirect_ops);
                prs->prs_mapping_memory += vdev_indirect_mapping_size(vim);
                indirect_vdev_id = vic->vic_prev_indirect_vdev;
        }

        return (0);
}

#if defined(_KERNEL) && defined(HAVE_SPL)
module_param(zfs_remove_max_segment, int, 0644);
MODULE_PARM_DESC(zfs_remove_max_segment,
        "Largest contiguous segment to allocate when removing device");

EXPORT_SYMBOL(free_from_removing_vdev);
EXPORT_SYMBOL(spa_removal_get_stats);
EXPORT_SYMBOL(spa_remove_init);
EXPORT_SYMBOL(spa_restart_removal);
EXPORT_SYMBOL(spa_vdev_removal_destroy);
EXPORT_SYMBOL(spa_vdev_remove);
EXPORT_SYMBOL(spa_vdev_remove_cancel);
EXPORT_SYMBOL(spa_vdev_remove_suspend);
EXPORT_SYMBOL(svr_sync);
#endif